1. Field of the Invention
This invention relates to a sheet transport mechanism having a flapper and, more particularly, to a sheet discharge mechanism provided in, for example, an image forming apparatus such as a printer or a copier, having a plurality of sheet discharge ports, and capable of changing over the discharge port by a flapper.
2. Related Background Art
Conventionally, for image forming apparatuses, e.g., printers, a mechanism is provided which includes a plurality of ports through which sheets of paper used for recording are discharged.
FIG. 1 schematically shows a section of an example of a conventional printer having a sheet transfer mechanism of this kind. As illustrated in FIG. 1, a toner image formed on the circumferential surface of a photosensitive drum 1 rotated in the direction of the arrow by an image forming means which is known per se is transferred to a sheet S which is pressed against the photosensitive drum 1 by a transfer roller 2. After receiving the toner image, the sheet S is transported while being guided by a sheet discharge guide 5a to a fixing device 3 to fix the toner image. The fixing device 3 is composed of a fixing roller 3a and a driven roller 3b. The driven roller 3b is pressed against the fixing roller 3a to rotate by following the fixing roller 3a. The sheet S is further transported to a pair of face-up (F/U) sheet discharge rollers 6 (which make the image of characters or the like face upward) while being guided by a sheet discharge guide 5b. The pair of F/U sheet discharge rollers 6 consist of a sheet discharge roller 6a and a driven sheet discharge roller 6b contacting and following the roller 6a.
On the downstream side of the pairs of F/U discharge rollers 6, a flapper (branch means) for changing the direction of discharge of the sheet S is disposed. When positioned as indicated by the double-dot-dash line, the flapper 7 guides the sheet S to the discharged sheet tray 9 to discharge the sheet S without inversion thereof. The sheet S is thereby placed on the tray 9 in a face-up position. When the flapper 7 is at the position indicated by the solid line, the sheet S discharged from the pair of F/U sheet discharge rollers 6 is guided by a guide member 10 so as to be inverted with respect to its two surfaces and is discharged onto a discharged sheet tray 12 by a pair of face-down (F/D) sheet discharge rollers 11 consisting of a sheet discharge roller 11a and a driven sheet discharge roller 11b contacting the roller 11a. The sheet S is thereby placed in a face-down position.
FIG. 2 shows another example of the conventional printer wherein an F/U sheet discharge roller 6a and a flapper (branch means) 13 having guide edges 13a positioned above and in the vicinity of the F/U sheet discharge roller 6a are disposed on the downstream side of the fixing device 3. The flapper 13 is rotatably supported by a support shaft 15.
Sheet S on which a toner image is fixed by the fixing device 3 is transported as described below. When the flapper 13 is at the position indicated by the solid line in FIG. 2, the direction of transportation is changed to the direction of the discharged sheet tray 9 by the flapper 13, and the sheet S is discharged onto the discharged sheet tray 9 via the gap between the F/U sheet discharge roller 6a and the flapper 13. When the flapper 13 is positioned by being turned counterclockwise from the position indicated in FIG. 2, the sheet S is guided by the flapper 13 and the guide member 10 so as to be inverted with respect to its two surfaces and is discharged so as to be placed on the discharged sheet tray 12 in a face-down position by the pair of F/D sheet discharge rollers 11.
In the above-described sheet discharge mechanism shown in FIG. 1, however, the pair of F/U sheet discharge rollers 6 are positioned on the downstream side of the fixing device and the flapper 7 for changing the direction of transportation of sheet S is disposed downstream of the rollers 6. The space occupied by the mechanism is therefore considerably large. This type of mechanism also entails the problem of a deterioration in transport performance owing to a change in the state of the pair of F/U sheet discharge rollers 6 with time due to wear thereof, since the F/U sheet discharge rollers 6 are always in contact with each other.
The sheet discharge mechanism shown in FIG. 2 is smaller than the mechanism shown in FIG. 1, thanks to the structure wherein no pair of F/U sheet discharge rollers corresponding to the above-described pair of F/U sheet discharge rollers 6 are provided on the downstream side of the fixing device 3. In this mechanism, however, the sheet transporting force applied from the F/U sheet discharge roller 6a is so small that the possibility of failure to discharge the sheet at the time of F/U sheet discharging is high.